To service a vessel at present, tackles are welded to the underbody of the vessel, pulleys and cables are hooked to these tackles and scaffolds are erected for removing or mounting propellers, propeller shafts and rudders or for servicing parts of the vessel such as stern tubes and the like. All these operations require many activities, the operators of which belong to various trades. For example, up to twelve different trades are involved in the dismounting and mounting of a propeller shaft. An adequate coordination of these activities is often difficult resulting in great loss of time and important manpower cost.